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with sequence variation in the CYP51 gene which encodes P45014DM. All of these
had F136 in P45014DM but isolates with two of the resistance levels, known as low
and high, had identical sequences of CYP51 . It was inferred that a second gene, very
closely linked to CYP51 , must be involved in differentiation between low and high
resistance. Isolates with a third phenotype, very high resistance, also had F136 but in
addition had a second mutation, from lysine to glutamine at residue 147 (K147Q).
Very high resistance co-segregated with the F136+Q147 allele of P45014DM but it is
not yet known if the K147Q mutation is directly involved in determining the very high
resistance phenotype.
Three factors complicate the picture further (Wyand and Brown, 2005). Firstly,
isolates of the wheat powdery mildew fungus, B. graminis f.sp. tritici with very high
resistance did not have the K147Q mutation, nor did they all share any other
mutation in P45014DM. Secondly, in a cross between triazole-resistant and sensitive
isolates of B. graminis f.sp. tritici , resistance co-segregated with the CYP51 allele of
the resistant parent, supporting the hypothesis that alleles of this gene confer triazole
resistance. The distribution of resistances, however, indicated that several genes
were involved in resistance, not just CYP51 . This suggests that B. graminis f.sp.
tritici has a polygenic or oligenic system controlling resistance to triazoles, in
addition to a major gene at the CYP51 locus. Finally, the fourth known phenotype of
triazole resistance in B. graminis f.sp. hordei , the medium level of resistance, had
the same sequence of CYP51 as triazole-sensitive isolates. Medium resistance is
controlled by a single gene unlinked to the high resistance phenotype (Brown et al.,
1996). Together, the genetic and molecular data indicate that medium resistance
must be controlled by a gene other than CYP51 .
Resistance to triazole fungicides and probably to sterol demethylation inhibitors
(DMIs) in general, of which triazoles are one type, therefore presents a complex
picture. Even in such closely-related fungi as B. graminis f.sp. hordei and tritici ,
there are rather distinct genetic systems of triazole resistance. Both involve CYP51,
but also different, uncharacterised genes, a single gene unlinked to CYP51 in
B. graminis f.sp. hordei and several genes, the segregation of which gives rise to a
continuous distribution of ED 50 s, in B. graminis f.sp. tritici . In Botrytis cinerea , by
contrast, resistance to DMIs is associated not with the target site of the fungicides
but with increased transport of the fungicide out of the fungal cell (Hayashi et al. ,
2001).
(c) Applications of fungicide resistance gene sequences
When they become widely-used, PCR-based tests will offer major advantages to
farmers as they cut the time needed to characterise the fungicide resistance of
isolates from several weeks to just a day or two. This will make it possible to give
rapid advice to farmers on whether or not a control failure was due to the pathogen
population being resistant to the fungicide or to some other cause. They will also
facilitate research on the population genetics of resistance, as it will be much quicker
and easier to distinguish resistant and sensitive isolates than it generally is
nowadays, given that pathology tests of fungicide resistance are often laborious and
expensive. The same warnings apply, however, as with the use of DNA sequences to
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